The present invention relates to an image scanning apparatus in which a sheet to be scanned is two dimensionally scanned with a light beam, and more particularly to an image scanning apparatus in which the main scanning is carried out with a deflector and the auxiliary scanning is effected by mechanically moving the sheet.
An image scanning method has been extensively employed in the facsimile field wherein a light beam is one dimensionally deflected by a deflector to form a scanning line on a sheet to be scanned (the main scanning) and the sheet is mechanically moved in a direction perpendicular to the scanning direction thereby to form a number of scanning lines in parallel with the scanning direction (the auxiliary scanning) to perform an image recording or reading operation. For an apparatus in which the image scanning operation is carried out as described above, it is essential that the main scanning is carried out regularly with a constant period and the sheet to be scanned is moved in the auxiliary scanning direction at a constant speed. If the speed of movement of the sheet in the auxiliary scanning direction changes, naturally the scanning lines become irregular in density, which appears as a change in image density; that is, the resultant image is considerably lower in quality. This is a serious problem in processing a continuous gradation picture. The deterioration of image quality is described in "Problems in the Auxiliary Scanning of Cylinder Type Facsimile", National Technical Report, vol. 22, No. 55, October 1976, pp. 550-558; and in "Visibility and Correction and Periodic Interference Structures in Line-by-Line Recorded Image", Journal of Applied Photographic Engineering, vol. 2, No. 2, April 1976, pp. 86-92, in detail. It has been found through experiments made by the present inventors that in recording or reading an image having a resolution ranging about 10 to 20 lines/mm, the variation not less than 0.1% in the movement speed of the sheet results in visually detectable variation in the density of the image obtained.
In a conventional auxiliary scanning mechanism for moving a sheet to be scanned at a constant speed with high accuracy, a sheet-to-be-scanned loading table is fixedly secured to a nut screwed on a threaded bar and the table is moved in the axial detection of the threaded bar at a constant speed by rotating the threaded bar at a constant speed. The auxiliary scanning mechanism using the threaded bar which is turned at the constant speed is considerably disadvantageous in that the operating speed cannot be increased very much, because it is necessary to return the table to the original position when a sheet has been scanned and the image scanning operation is suspended in order to return the table to the original position. Furthermore, in the auxiliary scanning mechanism, not only the threaded bar, nut and guide rail must be high in mechanical accuracy, but also a number of mechanisms such as a mechanism for placing a sheet to be scanned on the table, a mechanism for discharging the sheet from the table and a mechanism for positioning the sheet in place on the table are intricate. Thus, the auxiliary mechanism is high in manufacturing cost. In addition, because of the intricate mechanisms in combination, the sheet to be scanned is often loaded erroneously or jammed.
In view of the foregoing, the present inventors have developed image scanning apparatus in which the time required for replacing a sheet to be scanned is minimized, a sheet to be scanned will never be jammed, and the auxiliary scanning can be carried out at low cost with a high degree of accuracy. Such apparatus are the subject of U.S. patent application Ser. Nos. 395,320 filed on July 6, 1982 and 491,003 filed on May 3, 1983.
The apparatus as shown in FIG. 1 comprises an auxiliary scanning rotary drum 11 (hereinafter referred to merely as "a drum", when applicable), a light source 30 adapted to emit a main scanning light beam which scans a sheet to be scanned in the axial direction of the drum 11 which is conveyed by the drum, nip rolls 12 and 13 provided near the scanning position of the main scanning light beam, the nip rolls 12 and 13 being moved into and out of engagement with the drum; a pair of sheet supplying rolls 8 for delivering a sheet 1 to be scanned to the nipping region between the drum 11 and the nip rolls 12 and 13, a stopper 16 provided downstream of the drum 11 and the nip rolls 12 and 13 in such a manner that it goes in and out of the passage-way of sheets 1 which are supplied by the sheet supplying rolls 8, a pair of sheet discharging rolls 21 for discharging the sheet 1 which has been scanned, and control means (not shown) for controlling an operation of moving the nip rolls 12 and 13 into and out of engagement with the drum 11, and an operation of the sheet passageway, which are carried out in association with each other, and the rotation of the drum 11.
In the apparatus disclosed by the specification of U.S. patent application Ser. No. 395,320, the drum 11 is rotated counterclockwise in FIG. 1, and a sheet 1 to be scanned is supplied by the pair of sheet supplying rolls 8 when the nip rolls 12 and 13 are disengaged from the drum 11 and the stopper 16 enters the sheet passageway. When the sheet is set in place on the drum 11 with its front end supported by the stopper 16, the nip rolls 12 and 13 are engaged with the drum 11 to start the running of the sheet 1. In this operation the stopper leaves the passageway thus allowing the sheet 1 to freely run.
The above-described image scanning apparatus is advantageous in that recording or reading can be achieved with high density uniformity. However, it is disadvantageous in that, in the case where a sheet to be scanned is one liable liable to be damaged, it may be damaged or scratched when pushed against the rotating drum 11 by the nip rolls 12 and 13. In addition, the apparatus suffers from a problem in that as the scanning starting position is at a certain distance from the front end of a sheet to be scanned at all times the image reading or recording cannot be performed with high efficiency.
The apparatus disclosed by U.S. patent application Ser. No. 491,003 eliminated the above-described difficulties. In that apparatus, the drum 11 can be turned in both the forward and reverse directions. When the sheet 1 is run after it is pushed against the drum 11 by the nip rolls 12 and 13 with the front end of the sheet supported by the stopper 16, the drum is rotated in the reverse direction (clockwise in FIG. 1) so that the scanning starting position is moved towards the front end of the sheet and is then stopped near the front end. Under this condition, the drum is turned in the auxiliary scanning direction (counterclockwise in FIG. 1) to start the scanning operation.
The apparatus is advantageous in that the image reading or recording can be performed with high efficiency. However, it is disadvantageous in that, since controls must be provided for rotating the drum 11 selectively in the forward or reverse direction, the controls are rather intricate and switching the direction of rotation of the drum takes time.